9 comments:

As part of my ongoing argumentation with GW skeptics, I have been studying the calculations for the enhanced greenhouse effect. There are a lot of complications:1) the logarithmic effect2) multiple absorption lines to think about3) water vapor affecting some C-O2 lines4) absolute humidity reducing the impact of water vapor.

Have you seen anywhere where the entire calculation of the radiative forcing has been done, including the coefficient of d ln(concentration)?

The entire calculation requires a complex computer model. I have found simplified models such as this one, but I don't like them because they abstract out the physical effects such as how a greenhouse gas really works.

As far as the impact of carbon dioxide is concerned, the only absorption line of importance is the one at 15 microns. There is some overlap with water vapor here, as can be seen on the chart on this NASA page, which is actually about Richard Lindzen.

The idea that absolute humidity reduces the impact of water vapor makes no sense to me. More water vapor means more greenhouse effect (with a logarithmic increase of course).

There is a discussion at the bottom of this RealClimate page that is calling in question some of what I have written on my Greenhouse Effect page. If my understanding changes I will update the page.

Thanks for the reference to Archer: I think his chapter is useful, in that it does discuss the physics specifically. What I am trying to do is to think through enough of the physics to be clear on what would go into a model. My first interest is not in all the feedbacks, but rather in the question, "For a given delta in C-O2 concentration, what is the expected degree of radiative forcing?"

In thinking about this question for the enhanced greenhouse effect (EGH), I would normally ignore all lines for which water vapor is important, because even a substantial % change in C-O2 will not affect these cases.

However, intellectually it is more satisfying to have a framework which can evaluate not just the EGH situation (going from C-O2 = x to C-O2 = x + dx), but also the whole GH (going from C-O2 = 0 to C-O2 = x). Indeed, in order to address the ongoing complaints ("You GW guys are always forgetting about H2-O!"), it would be good to have a framework that can also go from y = 0 to y = (whatever it is) for water vapor as well.

(By a framework, I don't mean the whole complex model itself: I mean the intellectual structure and the equations that would allow one to construct such a model. Not the program, but what you tell the programmer to do.)

While thinking about this, I stumbled across a point that I had not seen addressed anywhere. (Maybe because it's not a practical issue.) If I think about two competing GH gases (call them A and B), I would normally conclude that if the concentration of A is very high, and B is very low, that a region with strong spectral lines of both will be rather insensitive to variations in B.

In detail: Because the EGH works by the increase in B elevating the height at which the optical depth for radiation of the given frequency reaches the value 1 (as measured coming down from space). That height is the height of the photosphere for that frequency; and when, by increasing B, that height is raised, the corresponding temperature of the photosphere is reduced. But if both A and B are involved in defining OD = 1, the impact of additional B will be very minor (because of the prevalence of A).

OK, you know this. But what I am wondering about is the question, "What if the vast bulk of the A molecules are confined to a level that is already below the OD = 1 point?" In that case, even though there's a lot more A than B, an increase in B will affect the OD = 1 point as though there were no A at all.

Of course, we normally make the assumption that the atmosphere is well-mixed, so if there is a lot of A, it will be well-represented throughout the atmosphere. However, in the case of water vapor, this should not be true: at higher elevations, as the temperature drops, more and more of the water vapor should condense out (the absolute humidity issue). So I'm sure that H2-O is not well-mixed in the atmosphere to the degree that C-O2 is.

Even in the less extreme case, where the relative concentration of A is not 0 above the OD = 1 point, but just reduced, there should be some increase in the impact of a delta in B on the temperature of the photosphere, and thus on the radiative forcing induced.

Does this give C-O2 a chance to be slightly more important? Does it affect the expected result for the 15-micron region, where both have strong lines?

Hi, Neal. Although there is some overlap with water vapor in the CO2 15 micron band, there is still plenty of room for CO2 to be effective. I don't think spectral overlap is a big issue.

You correctly point out that while CO2 is well mixed, water vapor concentration drops off rapidly with altitude. See for yourself with this on-line calculator. So it is an interesting question about what happens with spectral overlap if the water vapor is lower than the CO2.

The big issue is that CO2 is less than 0.04% of the atmosphere, while water vapor averages around 0.8%, a factor of 20. Add in that water absorbs over a wider spectrum, and CO2 looks trivial. So the question for me is where does the greenhouse effect take place? If it takes place in the upper atmosphere, as I assert in my Greenhouse Effect page, than CO2 becomes relatively more important.

I am right now trying to find the answers to some fundamental questions about how the greenhouse effect actually works. If you look at my page again, you will see a lot of the apparent certainty is now gone.

Maybe this is the place to ask a really basic question? On the answer to which I've missed in the first 'climate lesson' way back.

In all articles on the GH effect something along the line of 'the earth temperature must stay in balance' is simply stated and then left at that. I keep wondering 'why'? Why can't it warm up, or cool down? Why must it balance? Why must Earth radiate as much energy as it receives? Not the laws, but the underlying why? I suspect the answer is simple, but it's never given or explained - it's just stated.

Another related question is about energy. If the Earth has to radiate what it receives does that mean no work is done? Or that the work is in the heating of the Earth? OK, we have this 'we can't create or destroy' so is the reason we have to radiate what we receive because of that? Or why can't heat conduct into the Earth meaning the Sun warm it over time?

If there is an imbalance, that means that the temperature will continue to change. Since this is an attempt to calculate the new equilibrium temperature, by definition this implies that there is a radiative balance.

Hello.I would like to point out that you cite the vibrations of CO2 that are probably calculated (theoretical), not experimental.CO2 has vibrations at:667 1/cm infrared active1388 1/cm (doubly degenerate) Raman active2349 1/cm Raman active